1. Field of the Invention
The present invention relates to an x-ray tube of the type having a cathode and an anode which are arranged in a vacuum housing arrangement for magnetic deflection of the electron beam.
2. Description of the Prior Art
The possibility of deflecting the electron beam and thus the focal spot is particularly significant in connection with computed tomography, since by the known measure of displacing the focal spot between two end positions, an improvement of the image quality can be achieved therein by a multiplication of the data which are made available for the calculation of the image of a body slice.
German OS 41 25 926 and European Application 0 460 421 A1 disclose x-ray tubes of the above type. To avoid distortion of the focus geometry caused by the deflection of the electron beam, such distortions affecting the imaging quality, the magnetic field generated in the vicinity of the electron beam in the plane proceeding at a right angle to the propagation path of the electron beam must not have any notable gradients.
The x-ray tube taught by European Application 0 460 421 A1, in which the arrangement for deflecting the electron beam is formed by a deflection unit surrounding the shaft-shaped housing part, is not able to fulfil this requirement. Rather, the deflection unit effects not only a deflection, but also a defocusing of the electron beam. As a result of this effect of the deflection unit, the focal spot emerging at the point of impact of the electron beam on the incidence surface of the anode experiences not only a displacement on this anode surface, but also an undesirable change in size and/or shape.
In the x-ray tube described in German OS 41 25 926 the arrangement for deflecting the electron beam is formed by an air coil arranged outside the vacuum housing. In order to be able to fulfil the aforementioned condition, this air coil must disadvantageously be constructed with a very large volume. Besides this, to effect a defined deflection considerable electrical power must be fed to the air coil, so that in connection with the deflection of the electron beam, a high amount of dissipated heat is undesirably released, which presents another disadvantage in view of the thermal problems which already occur in the operation of x-ray tubes.
In "Elektronenstrahl-Technologie," Wissentschaftliche Verlagsgesellschaft mbH, Stuttgart, 1977 pages 89 to 95, Siegfried Schiller et al. teaches to deflect an electron beam by means of an electromagnet formed as a yoke with two legs connected by a base part, whereby the electron beam passes through the region between pole shoes provided at the ends of the legs. This does enable a deflection of an electron beam with low loss; however, the undesirable defocusing phenomena still occur in the deflection of the electron beam.